cc: Introduce BeginFrame sequence numbers and acknowledgements.

content/renderer/gpu/compositor_external_begin_frame_source.h

Index: content/renderer/gpu/compositor_external_begin_frame_source.h
diff --git a/content/renderer/gpu/compositor_external_begin_frame_source.h b/content/renderer/gpu/compositor_external_begin_frame_source.h
index 4b41f2a136f08d5e69762e599d29d73b151f457e..b37fa9e657ed5fd46c26ea9a3acf88cb8171816b 100644
--- a/content/renderer/gpu/compositor_external_begin_frame_source.h
+++ b/content/renderer/gpu/compositor_external_begin_frame_source.h
@@ -43,11 +43,12 @@ class CompositorExternalBeginFrameSource
   void AddObserver(cc::BeginFrameObserver* obs) override;
   void RemoveObserver(cc::BeginFrameObserver* obs) override;
   void DidFinishFrame(cc::BeginFrameObserver* obs,
-                      size_t remaining_frames) override {}
+                      const cc::BeginFrameAck& ack) override;
   bool IsThrottled() const override;
 
   // cc::ExternalBeginFrameSourceClient implementation.
   void OnNeedsBeginFrames(bool need_begin_frames) override;
+  void OnDidFinishFrame(const cc::BeginFrameAck& ack) override;
 
  private:
   class CompositorExternalBeginFrameSourceProxy